Electroacoustical apparatus



Patented Apr. 5, 1938 UNITED STATES ELECTROACOUSTICAL APPARATUS Leslie J. Anderson, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1936, Serial N0. 77,142

9 Claims.

This invention relates to electroacoustical apparatus, and more particularly to a microphone of the velocity type.

In such microphones, a conductor of small mass, such as a conductive ribbon, for example,

is disposed in a strong magnetic field to be actuated by sound wave pressure whereby an electromotive force is set up therein which may be conveyed to suitable amplifying apparatus. The

magnetic field may be set up by means of a permanent magnet or an electromagnet, and

heretofore, so far as I am aware, such magnets have been of the horseshoe or U-type and have been so arranged with respect to the conductive l5 ribbon as to provide a cavity which resonates at a relatively low frequency, thereby causing distortion in the neighborhood of that frequency.

Another objection to prior art microphones of this type is that the usual leads from the con- 20 ductive ribbon pick up energy inductively, and this also produces distortion. To overcome this objection, it has been proposed, as in the U. S. patent to Olson and Kreuzer, No. 1,897,732, to split the leads from one end of the conductive 5 ribbon and connect the split leads to the other end of the ribbon, whereby to provide a pair of loops with the ribbon. Since the two loops oppose each other, any energy picked up by them will be balanced out. In the microphone of the afore- 30 said patent, the two split leads are threaded through holes drilled in the pole pieces. This requires additional processing during manufacture, as well as adding to the cost of the microphone.

35 The primary object of my invention is to provide an improved microphone of the type described which will not be subject to the foregoing disadvantages.

More specifically, it is an object of my inven- 40 tion to provide an improved microphone of the type described which will be free from cavities that resonate at such frequencies as to cause annoyance and which will, also, require a minimum of processing during manufacture.

45 A further object of my invention is to provide an improved microphone as aforesaid which is compact in construction, inexpensive of manufacture, and highly efficient in use.

In accordance with my invention, I employ a 50 plurality of bar magnets which are disposed behind and extend across the conductive ribbon in such a manner that, in association with the pole pieces, they close off much of the space or cavity present in conventional microphones of the 55 velocity type. This not only raises the resonant frequency, or frequency of disturbance, of such cavity or cavities as maybe present in my improved microphone, but also decreases the amplitude of such disturbance, with the result that distortion is greatly diminished. Furthermore, 5 instead of drilling the pole pieces and threading the split leads therethrough, I utilize the supporting structure of the magnets and pole pieces to provide, with the conductive ribbon, a pair of balanced loops, and thereby greatly facilitate 10 manufacture, with accompanying decrease in cost.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment thereof, when taken in connection with the accompanying drawing, in which:

Figure 1 is a perspective view of one form of my invention as seen from the front andwith the screen removed, and

Figure 2 is a rear elevation thereof, with the casing members in section, showing particularly the relation of the bar magnets to the conductive ribbon and the supporting structure therefor. Referring more specifically to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown m a. mounting device I, such as a threaded socket member by means of which the microphone may be mounted on a suitable stand, having aflixed thereto a transformer housing or casing 3, preferably of iron. A substantially U-shaped bracket member 5 of non-magnetic but conductive material, such as brass, is secured to the housing 3 and constitutes the supporting structure for a pair of pole pieces 1, a plurality of bar magnets 9, and a transformer ll.

The pole pieces 1 are each L-shaped in cross section and are secured in opposed relation, one to each of the arms 5 and 5 of the bracket member 5, through one of the arms of the L, for example l The arms l of the L, which lie in a common plane, constitute the front faces of the pole pieces and have their adjacent edges slightly spaced from each other to provide an air gap therebetween in which a thin, crimped ribbon l3 of conductive material is movably mounted. Preferably,.each end of the ribbon I3 is tightly clamped between a non-conductive block or strip l5 secured to the faces 1* of the pole pieces I and a strip or block I! of conductive material, as by screws I8, the strips l1 serving as terminal members for making electrical connection with the ribbon l3 for a purpose shortly to be set forth.

It will be noted that the L-shaped pole pieces 1 and the conductive ribbon l3 combine to form a cavity behind the faces 1 and between the arms 5 and 5 of the bracket member 5. A cavity of this sort often produces a resonance at such a frequency as to be objectionable. To more or less close off or break up the cavity and at the same time to provide a magnetic field for the pole pieces 1, I mount the bar magnets 9 on the bracket member 5 between the arms I of the pole pieces I and in spaced relation to the conductive ribbon 3. The magnets 9 are preferably arranged in a plane parallel to that of the ribbonhl and may be either permanent magnets or electro-magnets. For ease in assembly, they may be provided with a central opening to receive a series of bolts I9 by means of which they are readily secured to the bracket member 5 simultaneously with the pole pieces 1. By breaking up the cavity in this manner, the frequency of disturbance is raised considerably and at the same time the amplitude of the disturbance is decreased, and as a result, my improved microphone is markedly free from distortion.

One end of the conductive ribbon I3, for example the upper end as seen in the drawing, is conductively connected to the ends of the arms 5 and 5* through its associated terminal strip I! and a pair of leads 2| and 22, respectively. The other end of the conductive ribbon I3 is concluctively connected, through its associated terminal strip H and a lead 23, to the primary coil of the transformer l I, this coil bein'g connected by a lead 25 to the base of the U-shaped bracket 5 at substantially its midpoint. Two opposed loops which balance out energy picked up inductively are thus formed with the conductive ribbon I3, one loop comprising the ribbon 13, the lead 2!, the arm 5*, the lead 25, the primary transformer coil, and the lead 23, and the other loop comprising the ribbon l3, the lead 22, the arm 5 the lead 25, the primary transformer coil, and the lead 23. The secondary coil of the transformer may be connected to a suitable amplifier (not shown) by means of a pair of leads 2'! and a cable 29. The conventional wind screen 3| of silk or the like may be placed around the bracket member 5 and the parts supported thereby, and the entire structure may be housed in a perforated metallic casing 33.

From the foregoing description, it will be evident that the bracket 5 not only constitutes a supporting member for the pole pieces and their associated magnets, but also advantageously provides part of the opposed or balancing loops whereby inductive pickup of energy is prevented. Although I have shown the bracket 5 as U-shaped, it will be apparent to those skilled in the art that it may be of any other shape, such as a V or an H, so long as it has a pair of arms which have a common point to which the lead 25 may be connected. Many other changes and modifications will, no doubt, readily suggest themselves to those skilled in the art. I desire, therefore, that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In sound translating apparatus, a substantially U-shaped bracket, a pair of pole pieces of L-shaped cross section each having one arm of the L fixed to an arm of said bracket in opposed relation and having their other arms in a common plane with the adjacent edges of said last named arms slightly spaced from each other to provide an air gap, a conductive ribbon supported for movement in said air gap and combining with said pole pieces to form a cavity, and a plurality of bar magnets carried by said bracket between said first named arms whereby to partially close off said cavity.

2. In sound translating apparatus, the combination of a bracket member having a pair 01' oppositely disposed arms of conductive material, a pair of pole pieces mounted one on each of said arms and slightly spaced from each other to provide anair gap therebetween, a movable conductor in said air gap, a pair of leads providing an electrical connection between one end of said conductor and each of said arms, and a third lead providing electrical connection between the other end of said conductor and a point common to each of said arms whereby to form a pair of opposed loops with said conductor.

3. In sound translating apparatus, the combination of a substantially U-shaped bracket member of conductive material, a pair of pole pieces mounted one on each of the arms of the U and slightly spaced from each other to provide an air gap therebetween, a movable conductor in said air gap, a pair of leads providing electrical connection between one end of said conductor and the ends of each arm of said bracket member, and a third lead providing electrical connection between the other end of said conductor and a point intermediate the ends of said bracket member whereby to form a pair of opposed loops with said conductor. I

4. The invention set forth in claim 3, characterized in that said third lead is constituted, at least in part, by the primary coil of a transformer associated with said apparatus.

5. In sound translating apparatus, a substantially U-shaped bracket'member of conductive material, a pair of pole pieces mounted one on each of the arms of the U and slightly spaced from each other to provide an air gap therebetween, a movable conductor in said air gap, and means forming with said conductor a pair of opposed loops for balancing out energy picked up inductively thereby, said loops being constituted in part by said bracket member.

6. In sound translating apparatus, a supporting member, a pair' of pole pieces carried thereby in spaced relation to each other to provide an air gap therebetween, a movable conductor in said air gap, and means forming with said conductor a pair of opposed loops for balancing out energy picked up inductively thereby, said loops being constituted in part by said supporting member.

'7. In sound translating apparatus, a supporting member, a pair of pole pieces carried thereby and having a pair of oppositely directed faces offset from said supporting member, the adjacent edges of said faces being slightly spaced from each other to provide an air gap therebetween, a movable conductor in said air gap, a plurality of bar magnets also carried by said supporting member in spaced relation to said faces and said conductor, and means including said supporting member forming with said conductora pair of opposed loops for balancing out energy picked up inductively thereby.

8. The invention set forth in claim '7 characterized in that said supportingmember is constituted by a U-shaped bracket member of which stituted by a pair of arms conductively coupled to each other, each of said arms having one of said pole pieces aflixed thereto, characterized further in that said pole pieces together with said conductor form a cavity between said arms, and characterized still further in that said bar magnets are mounted in said cavity whereby to at least partially close it off.

LESLIE J. ANDERSON. 

